Richard J. Silcox

Experimental Confirmation of a PDE-Based Approach to Design of Feedback Controls

Issues regarding the experimental implementation of PDE-based controllers are discussed in this work. While the motivating application involves the reduction of vibration levels for a circular plate through excitation of surface-mounted piezoceramic patches, the general techniques described here will extend to a variety of applications. The initial step is the development of a PDE model which accurately captures the physics of the underlying process. This model is then discretized to yield a vector-valued initial value problem. Optimal control theory is used to determine continuous-time voltages to the patches, and the approximations needed to facilitate discrete-time implementation are addressed. Finally, experimental results demonstrating the control of both transient and steady-state vibrations through these techniques are presented.

A PDE-BASED METHODOLOGY FOR MODELING, PARAMETER ESTIMATION AND FEEDBACK CONTROL IN STRUCTURAL AND STRUCTURAL ACOUSTIC SYSTEMS

A problem of continued interest concerns the control of vibrations in a flexible structure and the related problem of reducing structure-borne noise in structural acoustic systems. In both cases, piezoceramic patches bonded to the structures have been successfully used as control actuators. Through the application of a controlling voltage, the patches can be used to reduce structural vibrations which in turn leads to methods for reducing structure-borne noise. A PDE-based methodology for modeling, estimating physical parameters, and implementing a feedback control scheme for problems of this type is discussed. While the illustrating example is a circular plate, the methodology is sufficiently general so as to be applicable in a variety of structural and structural acoustic systems.

Modal coupling and acoustic intensity measurements

Acoustic-intensity measurement techniques for highly resonant coupled structural-acoustic systems are discussed. Frequency-averaging effects are shown to cause inaccurate results in such systems, and it is recommended that the analysis bandwidth be selected carefully to account for the response modal density and Q factor. The applicability of these findings to the analysis of reverberation chambers is indicated.

Direct adaptive predictive control using gradient descent

Predictive control has been successfully used for the regulation of plate vibrations. Past implementations of predictive control have included both feedforward and feedback algorithms. These algorithms may be used as a nonadaptive or an adaptive control method as long as conditional updating is employed or a sufficient level of dither is added to the control signals in order to maintain input and output data coherence for system identification. For large‐order systems with many input and outputs, the adaptation rate is severely limited due to the computational burden. The direct adaptive controller presented in this paper uses a gradient descent method to directly determine the predictive control parameters for the feedback regulator. The key features of this control scheme include the use of leakage to prevent drifting of the identified parameters in the presence of closed loop data. The initial information needed by the control scheme is an estimate of the plant order and an estimate of the plant pulse ...

Developments in active control of aircraft interior noise—Directions for tonal and broadband noise sources

Aircraft interior noise is a continuing problem for a wide range of aircraft types. Interior noise levels in general aviation aircraft make communication difficult, significantly impact pilot endurance, and are considered important to the reemergence of this class of aircraft. For passengers in propeller‐powered commuter aircraft, conversation is difficult at best and becomes oppressive on the passengers for long periods. It is also regarded as one reason for the replacement of propeller commuter aircraft by the emerging regional jet. In business jets, the passengers are often interested in conducting meetings during flights. This calls for low SPL and SIL for which the aircraft incurs both space and weight penalties. Large commercial aircraft pay a significant weight penalty for current interior noise levels considered acceptable. With the drive for more weight‐effective airframes, the challenge becomes even more difficult. This paper will show some current efforts to develop more efficient active contro...

Developments in aircraft interior acoustics—Recent progress and directions

Aircraft interior noise is a continuing problem for a wide range of aircraft types. High interior noise levels in general aviation aircraft make any communication difficult, significantly impact pilot endurance, and are considered important to the reemergence of this class of aircraft. For passengers in propeller‐powered commuter aircraft, conversation is difficult at best and becomes oppressive on the passengers for long periods. It is also regarded as one reason for the replacement of propeller commuter aircraft by the emerging regional jet. In business jets, the passengers are often interested in conducting meetings during flights. This calls for low SPL and SIL for which the aircraft incurs both space and weight penalties. Large commercial aircraft pay a significant weight penalty for current interior noise levels considered acceptable. With the drive for more weight‐efficient airframes, the challenge becomes even more difficult. This paper will overview current efforts to develop both design and nois...

The use of neural networks for optimum actuator grouping in time domain active control applications

Previous work has demonstrated the benefit of grouping actuators to decrease the number of degrees of freedom in an active control system. In this work, a time‐domain cost function was developed for on‐line actuator grouping and active structural acoustic control (ASAC) of a simply‐supported beam excited with a broadband disturbance. Actuators are considered grouped when their compensators are equal. Therefore, the cost function presented here incorporates a mean‐square error term related to the structure‐borne noise and an additional nonquadratic term which penalizes the controller for differences between respective compensator coefficients. The backpropagation neural network algorithm provides the proper procedure to determine the minimum of this cost function. The main disadvantage of using such a stochastic gradient technique while searching the prescribed control surface is converging to local minima. A resolution to this problem is discussed which incorporates using a variety of initialization condi...

Real time wave vector filtering in one‐dimensional media.

Often it is necessary to measure the wave vector components of propagating waves in vibrating media. Some examples are: measurement of traveling flexural and extensional waves in beams; measurement of acoustic waves in ducts; and determination of boundary conditions in structural or acoustic systems. In previous work, a time domain technique for wave vector filtering of plane waves in ducts was developed which required that the sensor spacing relative to the wavelength of interest be small. Frequency domain techniques have also been developed to perform wave vector filtering in both dispersive and nondispersive media. These techniques cannot be used in real time because of the signal processing requirements. In this work, a new time domain technique is presented that utilizes two sensor locations in conjunction with a digital filter, for each wave type, to provide a real time estimate of the complex amplitude of each of the traveling wave components for both dispersive and nondispersive media. The technique estimates the wave components over a band‐limited spectra whose limits are dictated by the spacing between the sensors (up to a spacing of half a wavelength). A simulation is used to determine the performance of the new technique in both dispersive and nondispersive media over a range of frequencies.

Active control of interior noise using piezoceramic actuators.

Active control of aircraft interior noise has recently received increased emphasis as an effective, lightweight noise control approach. Flight tests have demonstrated 10 to 15 dB of noise reduction using distributions of interior acoustic sources. However, to achieve control over the first three to five blade passage harmonics, actuator numbers ranging from 16 to 32 or more have been used. Additionally, the optimum distribution of actuators changes between those frequencies associated with the first one or two harmonics and the higher harmonics. Recent work at NASA Langley has investigated the use of piezoceramic patches bonded to the shell surface of a large‐scale composite fuselage model. These were used as force actuators in an active control system where the interior acoustic pressure was minimized using a least‐means‐squares algorithm. Results show an 8‐ to 15‐dB global noise reduction for a variety of test conditions. The results are compared for different actuator numbers, locations, and excitation...

Aircraft interior noise research at the NASA Langley Research Center

The Structural Acoustics Branch at the NASA Langley Research Center has been a major contributor to the field of aircraft interior noise research for the past 15 years. In addition to contributions from its in‐house program, the branch has supported many contributions from other organizations through an active grant and contract research program. Although the current emphasis of the program is on advanced turboprop airplanes and helicopters, the goal has always been to develop and improve interior noise prediction methodology. This allows for the incorporation of appropriate control measures in new aircraft at the design stage rather than through the use of add‐on acoustical treatments with potentially large weight penalties and reduced effectiveness. The proposed presentation will concentrate on some of the more recent basic research activities concerned with the transmission of noise into aircraft interiors through both airborne and structureborne paths. These include modeling of the structural response...